(1) Field of the Invention
The present invention relates to a pitch stabilizer and to a rotary-wing aircraft equipped with such a pitch stabilizer, particularly to minimize the attitude-hump phenomenon. Thus, the invention lies within the limited technical field of aircraft that are subject to the attitude-hump phenomenon.
(2) Description of Related Art
Traditionally, a helicopter includes, for example, a fuselage that extends longitudinally from a front end to a tail end on either side of a symmetrical antero-posterior plane and that extends in a vertical direction from a lower portion equipped with landing gear to an upper portion equipped with a main lifting and propulsion rotor.
The helicopter also has a tail rotor at its tail end. In particular, the function of the tail rotor is to counteract the yaw torque applied to the fuselage by the main rotor. This tail rotor also makes it possible to control the movements of the helicopter in yaw.
Helicopters sometimes also include additional stabilizer surfaces. For example, helicopters are customarily equipped with a surface that stabilizes continuous movements in yaw. Such a surface is located, for example, in the antero-posterior plane, so as to counteract the said yaw torque when the aircraft is operating at a high forward speed.
Such a surface that stabilizes yaw movements is generally known as a “tailfin” or “vertical stabilizer”.
Similarly, a helicopter sometimes has pitch stabilizer means that have an angle whose absolute value is between 0° and plus or minus 90° in relation to the said antero-posterior plane. These pitch stabilizer means include, for example, two pitch stabilizer surfaces extending symmetrically on either side of the said antero-posterior plane.
Such means for stabilizing pitch movements are sometimes referred to as the “horizontal empennage” or, more simply thereafter, as the “empennage”. The term “empennage” is more commonly used, because the stabilizer means are not necessarily horizontal. The phrase “pitch stabilizer means” is also used.
An empennage may consist of at least one traversing aerodynamic surface that passes from side to side transversely through the tail end of the aircraft, or, more preferably, at least one non-traversing aerodynamic surface that extends transversely from the said tail end.
Pitch stabilizer means are highly effective, with their effectiveness increasing in proportion to the forward speed of the helicopter. It is also acknowledged that the effectiveness of pitch stabilizer means is maximized when the wing area of the said means is maximized.
Nevertheless, the airflow over the main rotor of a traditional helicopter in flight is deflected downward, and, in some flight regimes, impacts the pitch stabilizer means, particularly during the transition to low speed, or even in hovering flight. This airflow thus applies forces to the pitch stabilizer means, and the pilot compensates for those forces by operating his flight controls.
However, when flight conditions vary, the deflection of the airflow is also changed. The same holds true for the forces applied to the pitch stabilizer means.
Those skilled in the art refer to this phenomenon as “attitude hump”.
In the transition regime and, for example, between 40 and 80 knots (kt), the forces applied by the airflow over the main rotor tend primarily to displace the empennage and, by impacting the pitch stabilizer means, cause the helicopter to adopt a nose-up attitude. This flight regime is usually known as a “transition regime”, because it usually occurs at low speed between a hovering flight regime and a cruising flight regime.
To balance the helicopter, the pilot must use his handle that controls the cyclic pitch of the main rotor blades in order to lower the nose-up attitude of the helicopter.
The resulting swerve degrades the performance of the aircraft. Furthermore, the shift of the aircraft into the nose-up attitude interferes with the pilot's visibility, especially in an approach regime.
Clearly, optimizing the pitch stabilizer means by maximizing the wing area of the said means simply accentuates the attitude hump.
Therefore, it appears impossible to use pitch stabilizer means having a large wing area on a helicopter without exacerbating the attitude-hump phenomenon.
The known remedies for this problem include stabilizer means equipped with a fixed aerodynamic stabilizer surface and a mobile aerodynamic surface. The position of the mobile surface in relation to the fixed stabilizer surface can then be controlled via the flight controls, so as to limit the attitude-hump phenomenon.
The term “stabilizer surface” is understood as referring to an elongated aerodynamic body, especially of the empennage type.
Although worthwhile, this architecture requires flight controls that lead to an increase in the mass of the aircraft, in addition to increasing the complexity of the aircraft.
A second solution consists of implementing pitch stabilizer means for a rotary-wing aircraft that are located outside the wake of the said rotary wing. However, the second solution may actually cause a significant reduction in the effectiveness of the stabilizer means, while also imposing implementation constraints.
Document FR 2962972 relates to a rotary-wing aircraft equipped with a fuselage that extends longitudinally from a front end to a tail end, and that is equipped with one upper pitch stabilizer means and at least one lower pitch stabilizer means. Each lower stabilizer means is positioned in the wake of the upper stabilizer means generated by an airflow passing over the rotary wing and impacting an upper surface of an upper stabilizer means.
The technological prior art also includes document FR 2966425, which describes an aircraft that includes an empennage and means for combating an aeroelastic flutter phenomenon.
Document FR 2167249 also describes an aircraft equipped with a V-shaped tail empennage.
Document U.S. Pat. No. 3,902,688 describes an aircraft equipped with a horizontal stabilizer located at the top of a vertical fin, and a horizontal stabilizer located at the bottom of this vertical fin.
Document WO 2004/007282 is also known.
Thus, the purpose of the present invention is to propose pitch stabilizer means for a rotary-wing aircraft, with a view toward minimizing the attitude-hump phenomenon.